Laser range finders may measure a distance to an object based on the propagation speed of light output from the laser. A typical laser rangefinder operates based on the time of flight principle, by sending a laser pulse in a narrow beam towards a target, and measuring the time taken by the pulse to be reflected from the target and returned to the sender. The propagation speed of light in air is relatively unaffected by temperature, humidity, etc. In contrast with sonar, a collimated laser beam can allow for measurements to a precisely known location. Such measurements using the speed of light can be very fast, but may also require fast detector electronics for accurate measurements.
Laser range finders may also operate based on the indirect time of flight or phase shift method, by sending a laser beam with sinusoidally modulated optical power to a target. Reflected light (from diffuse or specular reflections) is monitored, and the phase of the power modulation is compared with that of the sent light. The phase shift obtained may be 2π times the time of flight times the modulation frequency, illustrating that that higher modulation frequencies can result in a higher spatial resolution.